Power over Ethernet (POE) devices include power sourcing equipment (PSE) devices that deliver power and data through Ethernet cabling to one or more powered devices (PDs). PSE devices may be endpoint or mid-span devices, and the PDs at the other end of the Ethernet cabling include electronic devices, such as security cameras and other electronic devices, that receive operating power and data through the Ethernet cabling.
The Institute of Electrical and Electronics Engineers (IEEE) defined a POE Standard for delivering power to a PD on the same cable that is carrying data. The POE Standard may be found in IEEE 802.3 clause 33 as amended by IEEE Std 802.3 at-2009. The PSE provides power through the Ethernet cabling either in a common-mode on one of the data pairs (for 10/100/1000BASE-T) or on the spare wire pairs of the Ethernet cable for 10/100BASE-T applications. The POE Standard defines a PD Detection phase, an optional PD classification phase, a power-up phase, and an operating phase. During the detection phase, the PSE probes with limited current and voltage (in the range of 2.8V to 10V) to determine if the PD presents a signature resistance of approximately 25 kΩAccording to the POE Standard, a valid PD must present a resistance of between 23.75 kΩ and 26.25 kΩ.
After a valid PD signature resistance is detected, the PSE device may optionally perform a PD classification operation. In the PD classification phase, the PSE applies a voltage of between 15.5V and 20.5V to the network port and measures the current drawn by the PD coupled to the port through the Ethernet cable. The measured current determines the maximum power level that can be drawn by the PD during device operation, allowing the PSE to manage its power budget. If the PD classification operation is not performed, the default power level for the PSE may be set at 15.4 Watts.
After successful PD classification, assuming that the PSE has enough available power to satisfy the PD's needs, the PSE device enters a power-up phase. During the power-up phase, the PSE device applies power to the network port as long as the PSE device does not detect an overcurrent fault, a disconnection of the PD, or an input under-voltage or overvoltage fault. The POE+Standard specifies that, during the power-up phase, the PSE limits the current to a level that is between 400 mA and 450 mA for between 50 ms and 75 ms when the port voltage is more than 30V (IEEE 802.3 at 33.2.7.5).
Conventionally, PSE devices include a power field effect transistor (FET) that can be controlled to limit the current to the port. In an example, the port voltage (i.e., the voltage supplied to the network port) is the power supply voltage minus the voltage across the power FET. For some PSEs, the power supply voltage may be in a range of 50V to 57V, so when the port voltage is 30V, the voltage across the power FET is approximately 20V to 27V. Since the current can be reduced when the port voltage is less than 30V the worst case FET stress occurs at the point where the current is reduced which must be more than 20-27V and is typically 30V. Using typical numbers for a 30V power FET voltage with a 425 mA current for 60 ms, the power FET may see 12.75 Watts for 60 ms (approximately 0.765 Joules), which can produce significant heat. This amount of heat can be a determining factor in the selection of the power FET and the circuit layout, impacting both the cost and circuit density of the PSE circuitry.